Method of generating plasma having high ion density for substrate processing operation

ABSTRACT

An induction plasma source for integrated circuit fabrication includes a hemispherically shaped induction coil in an expanding spiral pattern about the vacuum chamber containing a semiconductor wafer supported by a platen. The windings of the induction coil follow the contour of a hemispherically shaped quartz bell jar, which holds the vacuum. The power source is a low frequency rf source having a frequency of about 450 KHz and a power in the range of 200-2000 Watts, and the pressure is a low pressure of about 0.1-100 mTorr. A high frequency rf source independently adjusts the bias voltage on the wafer.

This application is a continuation of application Ser. No. 08/273,574,filed Jul. 11, 1994, now U.S. Pat. No. 5,405,480, which is acontinuation-in-part of application Ser. No. 07/971,363, filed Nov. 4,1992, now U.S. Pat. No. 5,346,578.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to plasma sources, and more particularlyto induction plasma sources for integrated circuit fabrication.

2. Description of Related Art

Plasma etching is useful in many fields, but particularly in the fieldof microelectronic fabrication. Microelectronic fabrication ofintegrated circuits requires mass replication of tightly controlled,submicron-sized features in a variety of materials, as well as selectiveremoval of material without causing damage to sensitive structures andremaining materials. Illustrative application of plasma etching tomicroelectronic fabrication of integrated circuits includes ion sputtercleaning, dielectric gap filling involving simultaneous chemical vapordeposition ("CVD") and etch, chemical blanket etchback without resist,and chemical pattern etch with resist.

Various plasma source methods and geometric designs for reactors areknown for use in plasma deposition and etching. For example, electroncyclotron resonance ("ECR") sources are available from Applied Scienceand Technology, Inc., Woburn, Mass. and from the WAVEMAT Corporation,Plymouth, Mass. Also, wafer cleaning and etching processes have commonlybeen done with equipment using various cylindrical shaped quartz vesselsof various diameters having induction windings of various pitches,either constant or variable, as well as with flat spiral inductionwindings mounted above the dielectric chamber top plate. Radio frequency("rf") diode and triode configurations are also known in which the waferelectrode and possibly other electrodes are powered at 13.56 MHz toproduce the plasma.

Physical sputtering, one of several plasma etching mechanisms, involvesthe removal of material by energetic ions, which cross the sheath andtransfer energy and momentum to the material being etched. Asimplemented on a great many of the prior art inductively coupled cavityand/or diode and triode machines, physical sputtering suffers from anumber of disadvantages, including low material removal rate, poor etchuniformity due to poor ion current uniformity, and electrical damage tothe substrate from ion bombardment and implantation due to high ionenergies. The ECR sources provide improved performance, but withconsiderably greater complexity than induction-type sources.

Hence, a need continues for plasma source systems that are able toprovide good ion density to achieve high etch rates, ion currentuniformity to achieve uniform removal of material over large diametersubstrates, and operational stability at low pressure to achieve a moreuniform ion distribution in the plasma and better directionality of ionsin high aspect ratio structures, all in a generally simple machineimplementation.

SUMMARY OF THE INVENTION

The present invention achieves high ion density, good ion currentuniformity, and stable low pressure operation in a generally simplemachine implementation.

In one embodiment of the present invention, an induction plasma source,the induction coil is hemispherical in shape. A chamber into which asubstrate may be introduced is disposed inside the induction coil. In afurther embodiment, the induction coil follows the contour of ahemispherically shaped vessel, which contains the chamber. In yet afurther embodiment, the power source is a low frequency source having afrequency of about 450 KHz and a power in the range of 200-2000 watts,and the pressure is a low pressure of about 0.1-100 mTorr.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section view of a hemispherical induction plasmasource and related components of a plasma etch system.

FIG. 2 is a perspective cut-away view of the hemispherical inductionplasma source of FIG. 1.

FIG. 3 is a plan view of the connection between the hemisphericalinductor and the rf matching network of FIG. 1.

FIG. 4 is a plan view of the connection between the hemisphericalinductor and the plasma chamber of FIG. 1.

FIG. 5 is an equivalent circuit of the hemispherical induction plasmasource of FIG. 1.

FIG. 6 is a graph of etch rate versus ion source power for thehemispherical induction ion source of FIG. 1.

FIG. 7 is a graph of ion current uniformity for the hemisphericalinduction ion source of FIG. 1 and a standard diode etch apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A hemispherical induction plasma source 1 is shown in cross-section inFIG. 1 and in a simplified perspective in FIG. 2. The hemisphericalinduction plasma source 1 is contained within a stainless steel sourcehousing 10 measuring 26.62 cm high and 43.82 cm wide, and includes ahemispherical induction coil 18 that is provided in an expanding spiralpattern on four winding forms--only forms 12 and 14 are illustrated forclarity. Four forms are used to simplify assembly of the induction coil18, although other form types such as a unitary form are suitable aswell, depending on the manufacturing techniques employed. The windingforms, e.g. forms 12 and 14, are made of any suitable material, whichinclude dielectric materials such as nylon. The induction coil 18 isheld in place inside channels in the winding forms, e.g. 12 and 14, byany suitable dielectric strapping, adhesive, or cement. Particularadvantages in temperature control of a manufacturing process areachieved by holding the induction coil 18 in place using a thermallyconductive material. One thermally conductive material is a thermallyconductive elastomer, for example Sylgard® Q3-6605 Thermally ConductiveElastomer, which is available from Dow Corning Corporation, Midland,Mich. The winding forms, including forms 12 and 14, are securelyattached to the housing 10 in any convenient manner, as by bolts oradhesive.

The induction coil 18 is copper tubing having an inner diameter of 3.0mm and an outer diameter of 4.75 mm. The hemispherical induction coil 18has a radius to centerline of 7.75 cm. The expanding spiral pattern ofthe induction coil 18 is made of thirty-six windings. The first windingis nearly coplanar with substrate 32 and each subsequent winding spiralsupward with a 2.432 degree angular displacement for a total of 36 coils.

During processing operations, the induction coil 18 is positioned abouta vacuum chamber 30 contained within a quartz vessel or bell jar 20, inconjunction with a stainless steel chamber top plate 24 of any suitablethickness, illustratively 1.91 cm thick. Preferably, the vessel 20 ishemispherically shaped so that there is a balanced coupling of rf(uniform dielectric spacing) into the vacuum cavity. Generally, thevessel material is an insulating dielectric having sufficient structuralintegrity to withstand a vacuum. Suitable materials include quartz,PYREX glass, aluminum oxide (Al₂ O₃, also known as sapphire), polyamide,and other oxide or nitride composites. Illustratively, the radius of thevessel 20 is 17.78 cm, and the vessel material is quartz having athickness of 0.51 cm. The induction coil 18 follows the hemisphericalcontour of the vessel 20, which is capable of holding a vacuum andcontains the substrate, illustratively a semiconductor wafer 32containing integrated circuit chips in fabrication.

The housing 10 is mounted onto the chamber top plate 24 in anyconvenient manner. FIG. 1 shows the housing 10 as being engaged by an rfseal 22, which includes copper leaves to prevent spurious rf emissionsfrom the induction plasma source 1.

The semiconductor wafer 32, illustratively 200 mm in diameter, issupported within the chamber 30 by an electrically conductive (e.g.stainless steel) wafer support pedestal 42 that includes a platen 40having a stainless steel portion 44 underlying the wafer 32 and aceramic dark space ring 46 extending beyond and in the plane of theplaten portion 44. The diameter of the portion 44 is 18.35 cm, and theouter diameter of the dark space ring 46 is 28.62 cm. Under the platen40 is a dark space shield 50, which has an outer diameter of 20.32 cm.

The pedestal 42 is capable of vertical motion, which is imparted by anysuitable mechanism (not shown). The position of the pedestal depends onwhether the plasma etch system is operating in process mode or in wafertransfer mode. In process mode, the platen 40 is positioned within thechamber 30, as shown in FIG. 1. Bellows 52, which is provided to isolatethe mechanical components of the pedestal drive system at atmosphericpressure from the vacuum in chambers 30 and 60, is well extended. Thewafer 32 rests on the pedestal 40, within the process chamber 30.

For wafer unloading and loading, the pedestal 40 is lowered into a wafertransfer region 60, which is 7.54 cm in height and includes at one end asealable wafer transfer opening 26 having a height of 4.60 cm. Thebellows 52 is well compressed, and three lifter pins--only pins 54 and56 are shown--protrude through holes (not shown) in the platen 40 so asto support the wafer 32 in a stationary position within the transferregion 60 as the pedestal 42 lowers. The sealable wafer transfer opening26 is provided to permit a wafer transport arm (not shown) access beyondthe wafer transfer flange 28 into the transfer region 60 during wafertransfer mode. Suitable wafer transport arms and associated mechanismsare well known in the art. In a wafer transfer operation, tines on theend of the transport arm are inserted under the wafer 32 as it issupported by the lifter pins (e.g. pins 54 and 56). The transport arm israised to lift the wafer 32 off of the lifter pins, so that when thetransport arm is retracted, the wafer 32 is removed from the transferregion 60. A new wafer is substituted on the tines, and the transportarm is then moved into a position over the lifter pins (e.g. 54 and 56).The transport arm is lowered to deposit the wafer 32 onto the lifterpins, and then withdrawn. The pedestal 42 is raised to cause the wafer32 to be deposited on the platen 40.

The induction source 1 is suitable for use in a variety of applications,including ion sputter clean, chemical blanket etchback, chemical patternetch, and plasma-enhanced chemical vapor deposition ("PECVD"). Ionsputter clean involves the use of a plasma obtained from a suitableinert gas such as argon at low pressure to remove material from thesurface of the substrate by momentum transfer. In the illustrativearrangement of the induction system 1 for etch clean shown in FIG. 1,argon gas is introduced into the chamber 30 through a single port 58(FIG. 1) located in the chamber sidewall just below the platen 40.Chemical etching uses reactive gases instead of an inert gas intypically a higher pressure regime than an ion sputter clean, and issuitable for etchback or for pattern etch where a photoresist or othermasking material is present. Because of the higher pressure or greaterreactivity of species, an arrangement of the induction system forchemical etching (not shown) preferably uses a symmetrical multiple portarrangement about the substrate for introducing the reactive gases.PECVD uses different reactive gases that induce film deposition. Anarrangement of the induction system for PECVD (not shown) preferablyuses the symmetrical multiple port arrangement. When used with carefulsubstrate bias control, the induction system for PECVD is suitable fordielectric gap filling.

A vacuum system (not shown) of any suitable type is connected to thetransfer region 60 for evacuating the chamber 30. Suitable vacuumsystems are well known in the art. After chamber 30 is evacuated,process gas, which for an ion sputter clean is preferably argon, isfurnished to the chamber 30 through the port 58 to attain a desiredpressure of process gas. Illustratively for an ion sputter clean,sufficient argon is introduced to establish a low pressure in the rangeof about 0.1-100 mTorr, and preferably 0.1-10 mTorr.

The radio frequency ("rf") subsystem of the induction plasma source 1includes matching capacitors 6 and 8, which are enclosed within astainless steel rf match enclosure 2. Capacitors 6 and 8 are correctedto bus bars (only bus bar 4 shown), and the assembly is mounted onto adielectric block 5 which is mounted on the housing 10.

The induction coil 18 is coupled to the rf matching network capacitors 6and 8 as shown in the detail of FIG. 3. Capacitors 6 and 8 each have oneterminal screwed into the copper bus bar 4 and the other terminalscrewed into copper bus bar 204. Bus bar 4 is connected to the lowfrequency source 410 (FIG. 5). Bus bar 204 is connected to end 206 ofthe copper tubing of which the induction coil 18 is formed throughfitting 208. Fitting 208 is screwed into a channel through the bus bar204. Another fitting 210 is screwed into the other end of the channel.Teflon tubing 212 is connected to the fitting 210 for delivering acooling fluid. The induction coil 18 is coupled to the grounded topplate 24 as shown in the detail of FIG. 4. Bus bar 302 is bolted to thehousing 10 by bolt 304, and is connected to end 306 of the copper tubingof which the induction coil 18 is formed through fitting 308. Fitting308 is screwed into a channel through the bus bar 302. Another fitting310 is screwed into the other end of the channel. Teflon tubing 312 isconnected to the fitting 310 for draining a cooling fluid.

The rf subsystem of the induction plasma source 1 is represented in FIG.5. The power source includes a low frequency source 410 and a highfrequency source 420. The low frequency source 410 has a frequency ofabout 450 KHz and a power in the operating range of 200-2000 watts. Thelow frequency source 410 is connected to the induction wiring 18 througha low frequency matching network that includes the capacitors 6 and 8,connected in parallel. The low frequency matching network is tuned tocouple low frequency rf energy into the plasma cavity in accordance withthe cavity shape, pressure, and plasma chemistry, in a manner well knownin the art. Illustratively in this embodiment, capacitors 6 and 8 aretransmitting-type mica capacitors, each having a value of 1200 pf at6000 rated volts. The high frequency source 420 has a frequency of about13.56 MHz and a power in the operating range of 25-500 watts. The highfrequency source 410 is connected to the platen 40 through an autotunehigh frequency matching network 422 such as, for example, model AM-5,available from RF Plasma Products, Inc. of Kresson, N.J. The autotunematching network 422 has an internal dc bias control that allows aregulated bias voltage at the wafer 32 to be achieved.

The induction coil 18 is cooled by any suitable liquid, an example ofwhich is chilled water. Chilled water is introduced to the inductioncoil 18 from source 440 through valve 442, and is returned to sink 446through flow switch 444. Illustratively, the chilled water is deliveredat a pressure in the range of 3-5 bars, for example.

During fabrication of integrated circuits, the vessel or jar 20 enduresthermal cycling. A chamber vessel is constructed from an insulatingdielectric material so that cooling of the chamber is difficult andheating is significant. Temperature increases of 300° F. to 400° F. aretypical in a conventional chamber, which tends to cause the insulatingdielectric material to delaminate or flake, disadvantageously resultingin particulate generation in the chamber.

However in the illustrative embodiment of the chamber 30, the coolingeffect of the cooling fluid is augmented when the induction coil 18 ispacked ("potted") in a thermally conductive material having a highdielectric strength so that both the coil 18 and the quartz vessel orbell jar 20 are cooled. Temperature control of the quartz vessel or belljar 20 is improved and temperature excursions are typically reduced toless than 100° F. in continuous operation.

Although the induction plasma source 1 is mechanically uncomplicated andrelatively simple to manufacture, it advantageously achieves high iondensity, good ion current uniformity, and stable low pressure operation.When used for argon ion sputter cleaning, the uniform high density ionflux generated by induction plasma source 1 provides, in combinationwith a suitable high frequency bias applied to the wafer 32, a gentle,low voltage argon sputter particularly suitable for the removal of thinoxides and contaminants prior to the deposition of thin metal films.This gentle, low voltage argon ion sputter clean avoids gate damage thatcould occur during contact cleaning at higher energies. The argon ionsputter clean is also advantageously used to remove thin native oxidesin contacts and vias, including those down to the silicon substrate,which provides for decreased contact resistance to the first level layerdue to the removal of native oxide as well as the reduction in theamorphization of the silicon surface.

Illustrative characterization data is presented in FIGS. 6 and 7. FIG. 6is a graph of etch rate as expressed in angstroms/minute versus ionsource power as expressed in watts for various wafer bias voltages(V_(dc) =-50 V, -75 V, -100 V), the hemispherical induction plasmasource 1 operating on thermal oxide wafers at a pressure of 0.6 mTorr ofargon. The actual sheath potential (total voltage scene at the wafersurface) is approximately 30 volts greater than the wafer bias voltage.FIG. 6 shows that high etch rates are achieved at very low voltagelevels. At 200 watts ion source power (source 410), an etch rate ofabout 250 Å/min is achieved for a wafer bias voltage of -100 volts,decreasing only to about 200 Å/min (about 20% decrease) at a wafer biasvoltage of -50 volts (50% decrease). At 400 watts ion source power, anetch rate of about 600 Å/min is achieved for a wafer bias voltage of-100 volts, decreasing only to about 450 Å/min (about 25% decrease) at awafer bias voltage of -50 volts (50% decrease). At 600 watts ion sourcepower, an etch rate of about 950 Å/min is achieved for a wafer biasvoltage of -100 volts, decreasing only to about 700 Å/min (about 26%decrease) at a wafer bias voltage of -50 volts (50% decrease). At 800watts ion source power, an etch rate of about 1200 Å/min is achieved fora wafer bias voltage of -100 volts, decreasing only to about 900 Å/min(about 25% decrease) at a wafer bias voltage of -50 volts (50%decrease).

FIG. 7 is a graph expressing ion current uniformity as a function of ioncurrent in milliamperes versus radial position in centimeters. Curve 602represents the ion current across a wafer in a sputter clean processusing the hemispherical induction ion source of FIG. 1 at a pressure of0.6 mTorr of argon and a wafer bias voltage of minus 100 volts. Ioncurrent varies only about +/-1.1%. Not only is the ion current above thepedestal 40 uniform, but the ion current is quite high at about 6 mA,even despite the low pressure operation at 0.6 mTorr. Curve 604represents the ion current across a wafer in an sputter clean processusing a conventional diode etch at a pressure of 20 mTorr and a waferbias voltage of minus 700 volts. Ion current varies about +/-12.9%.

Hence, when used with dual power supplies to independently control theion-current and the ion energy at the wafer surface, the hemisphericalinduction plasma source 1 allows high etch rates to be achieved whileminimizing the total voltage seen at the wafer 32. High ion densities ofgreater than 1×10¹¹ (ions/cm³) yield excellent etch rates of greaterthan or equal to 300 angstroms/minute for thermal oxides at total wafervoltages of less than 150 volts.

To understand the reasons for the significantly improved resultsachieved by the relatively simple hemispherical induction plasma source1, consider first the general principle that both gas pressure andfrequency have an effect on sheath potentials. As pressure is loweredbelow about 50 or 100 mTorr, the sheath thickness and voltage across thesheath begin to increase in many plasma systems from tens of volts tohundreds of volts or more, as typified by the diode etchcharacterization shown in curve 604 of FIG. 7. Correspondingly, theplasma potential goes up and the ion-substrate bombardment energy risessharply with decreasing pressure. These effects are a consequence oflonger mean free paths and reduced collision rates between electrons andmolecules. Electron energy and potentials increase in order to raise theprobability of ionization and sustain the plasmas, despite the lowercollision rates. Lower rf excitation frequency has a similar influenceon diode systems. When gas pressure is in the range of about 100 to 1000mTorr and frequency is lowered from about 10 MHz to less than about 1MHz, once again sheath potentials increase dramatically and favorenergy-driven ion assisted etching. The potential increase is attributedto a change in the plasma sustaining mechanism.

Hence, in general terms for diode systems, frequency and pressure areinterchangeable variables, and either low frequency or low pressureincreases sheath potentials, which helps sustain the plasma, but whichalso tends to damage the substrate and cause gate damage in MOSFETdevices. For this reason, a more efficient method of ionization shouldbe employed.

The hemispherical induction source 1 achieves a gentle, low voltage,high rate argon ion sputter clean at low pressure that avoids gatedamage that could occur during contact cleaning at the higher energiestypically observed with other plasma sources. We believe that theadvantageous performance characteristics of the hemispherical inductionplasma source 1 can be explained as follows. Induction discharges aresustained in the induction plasma source 1 during low pressure operation(e.g. 0.6 mTorr and below) by the induced azimuthal electric field nearthe wall of the vessel 20. The oscillating current in the excitationcoil produces an axial, time-varying magnetic field which induces anoscillating azimuthal electric field. Electrons entering the regionwithin the plasma boundary near the induction coil 18 are acceleratedazimuthally and produce an electric current opposite to that in theinduction coil 18. This results in the axial magnetic field being mostlycancelled in the interior of the plasma discharge. Consequently, themagnitude of the electric field is very low in the interior of thechamber 30.

The exterior part of the plasma near the induction coil 18 is the mainregion in which energy is transferred to the plasma electrons. Theseelectrons rapidly diffuse through the plasma volume due to the elasticscattering collisions with the atoms of the gas. The electrons, whichhave energy of about 20 eV, make ionizing collisions with atomsthroughout the chamber 30. In the induction plasma source 1, at gaspressures well below a milliTorr (e.g. 0.6 mTorr) the likelihood is thatabout 1 collision per ion occurs from creation of the ion to when ithits the wall after a mean free path of average length of about 10 cm,which is essentially a free fall to the wall. Contrast the low pressurecase from the case at higher pressures of several milliTorr, when morethan 10 collisions occur over such a path length. In this case, the ionsdiffuse to the wall.

The spatial dependence of the ionization rate is highly dependent on thepressure and electron mean free path for scattering, since the electronsare energized at the outer wall near the induction coil 18. When thepressure is very low (0.6 mTorr), the mean free path for the electronsis such that they normally transit the vessel without a collision.However, the path length of the electron in the inductive field isincreased more than 100 times that of an electron moving in the electricfield of a diode (parallel plate) system of comparable size. Theinductive source also allows ionization events to occur in a very largevolume (at the perimeter) which contrasts to the diode ionizationmechanism which occurs mostly at the sheath boundary through "waveriding" and secondary injection. One might explain the very uniform ioncurrent above the electrode as follows. The likelihood of an electronmaking ionizing collisions is smaller in the center than nearer thewalls since they need not pass through the center. Nonetheless, sincethe height of the plasma column above the center is greatest, thecurrent falling on the center of the electrode is nearly equal to thatfalling on the edge where the plasma column is not so tall but where theionization rate is larger.

The sputter rate is dependent on the high frequency bias voltage. Theobserved sputter rates for, e.g., SiO₂ correlates well with the ioncurrent density because the sheath potential is quite constant over thesurface of the wafer 32 and the sputter rate is simply the product ofthe ion current density and the sputter efficiency, which is a functionof the ion energy. The sheath potential is shown to be uniform becausethe time averaged potential in the plasma just above the electrodesurface of the pedestal 40 (at about 1.9 cm) is constant over the areaof the pedestal electrode, while the potential on the pedestal electrodesurface itself is independent of position. The sheath potential,therefore, which is the difference of the plasma and electrode surfacepotentials, is also essentially uniform.

While the invention has been described with respect to the embodimentsset forth above, the invention is not necessarily limited to theseembodiments. Accordingly, other embodiments, variations and improvementsnot described herein are not necessarily excluded from the scope of theinvention, which is defined by the following claims.

What is claimed is:
 1. A method of generating a high density of ions fora substrate processing operation, comprising:placing a substrate in aprocess chamber; introducing a process gas into the process chamber;generating a bias voltage on the substrate; providing an oscillatingcurrent to an induction coil defining in conjunction with the substratea column of the process gas, the oscillating current in the inductioncoil establishing a strong axial time-varying magnetic field in theprocess gas column that accelerates electrons azimuthally inside of andproximate to the induction coil, the accelerated electrons creating ahigh electric field region in the process gas column inside of andproximate to the induction coil and mostly canceling the axial magneticfield inside of the high electric field region, thereby creating a lowelectric field region in the process gas column inside of the highelectric field region; and maintaining the process gas in the processgas column at a pressure so that electron collisions in the high and lowelectric field regions ignite a plasma and cause an ion current to fallon the substrate, the induction coil being shaped so that portions ofthe substrate nearer to the induction coil receive a greatercontribution of ion current from ionization in the high electric fieldregion than portions of the substrate farther from the induction coil toequalize ion current falling across the substrate.
 2. A method as inclaim 1 wherein the maintaining step comprises maintaining the processgas in the process gas column at a pressure of about one milliTorr orless.
 3. A method as in claim 1 wherein the maintaining step comprisesmaintaining the process gas in the process gas column at a pressure ofabout 0.6 milliTorr or less.
 4. A method as in claim 1 wherein theoscillating current is a low frequency oscillating current, and the biasvoltage generating step comprises applying high frequency radiofrequency power to the substrate.
 5. A method as in claim 4 wherein:thesubstrate processing operation is an ion sputter process; the processgas is an inert gas; and the pressure maintaining step comprisesmaintaining the process gas in the process gas column at a low pressure.6. A method as in claim 4, wherein:the substrate processing operation isa chemical etching procedure; the process gas introducing step comprisesintroducing a reactive gas to the substrate from multiple gas sourcesarranged radially symmetrical thereabout; and the maintaining stepcomprises maintaining the process gas in the process gas column at ahigh pressure.
 7. A method as in claim 1, wherein:the substrateprocessing operation is a plasma-enhanced chemical vapor depositionprocedure; and the process gas is a reactive gas.
 8. A method as inclaim 7 wherein the pressure maintaining step comprises maintaining theprocess gas in the process gas column at a high pressure.
 9. A method asin claim 8, wherein the process gas introducing step comprisesintroducing the reactive gas to the substrate from multiple gas sourcesarranged radially symmetrical thereabout.
 10. A method as in claim 1,further comprising the step of evacuating the chamber to a pressure lessthan atmospheric pressure prior to the pressure maintaining step.